8 Prof. Rutherford and Miss Brooks : Comparison of 



Radium. — Two different specimens were employed. One 

 was of impure radium chloride kindly presented to me 

 by Elster and G-eitel two years ago ; this did not give 

 off any emanation and only a small proportion of de- 

 flectable rays. The other, from P. de Haen, Hannover, 

 was not very strong in deflectable rays, but gave out a 

 large amount of emanation when slightly heated. 



In the course of the paper we shall examine the following 

 types of radiation. 



A. Uranium. 



(1) The a or early absorbed radiation. 



(2) The ft or deflectable rays. 



B. Thorium radiations. 



(3) The simple radiation given out by a thin layer. 



(4) The radiation from the " emanation." 



(5) The excited radiation. 



C. Polonium. 



(6) Simple radiation. 



D. Radium. 



(7) Radiation not affected by a magnetic field. 



(8) Radiation from the emanation. 



(9) Excited radiation. 



(10) Magnetically deflected rays. 



Absorption of Radiation by Metals. 



In examining the absorption of the radiation by meral-foil 

 and other substances, the apparatus shown in fig. 2 was 

 employed. The active compound in the form of fine powder 

 was uniformly spread over a shallow depression 6*5 cm. 

 square in a large lead plate. This corresponds to plate A in 

 fig. 2. The rate of leak was observed by means of an elec- 

 trometer between plates A and B with, if necessary, a 

 suitable capacity in parallel. The plate A was connected 

 to one pole of a battery of 300 volts, the other pole of which 

 was earthed. Preliminary experiments showed that with 

 this voltage the maximum or saturation current between 

 the plates was obtained for all the radioactive substances 

 examined. 



In most of the experiments described in this paper an 

 Ayrton electrometer was used. In some of the later experi- 

 ments, however, the White pattern of Kelvin electrometer 

 was used. The former electrometer could be readily arranged 

 to give 200 mm. divisions for 1 volt P.D. between the 



